1. Field of the Invention
The present invention relates to a pulse-type driving device, and more particularly to one which is used in driving infrared diodes on a mouse or track ball.
2. Description of the Prior Art
In a pointing input device such as a mouse or a track ball, photoelectric components such as a pair of phototransmitter and photoreceiver are used to detect displacement of the pointing input device in an X coordinate and a Y coordinate. A conventional control circuit as shown in FIG. 5 comprises a pointer control circuit 70 which has four terminals XA, XB, YA, YB respectively connected to one of four detecting circuit 80, 90, 80' and 90' where two detecting circuits 80 and 90 are used for detecting displacement along the X coordinate and two detecting circuits 80' and 90' identical to the detecting circuits 80 and 90 are used for detecting displacement along the Y coordinate. For example, the detecting circuit for the X coordinate comprises two light emitting diodes 81, 91, two phototransistors 82, 92 and two waveform-shaping transistors 83, 93. The light emitting diodes 81, 91 and the phototransistors 82, 92 are positioned at an opposite side to a photogrid wheel (not shown) thus cooperating to detect the displacement in the X coordinate. Similarly, to detect the displacement in the Y coordinate, another photogrid wheel is provided to cooperate with the light emitting diodes 81', 91' and the phototransistors 82', 92'
The collectors of the phototransistors 82, and 92 are connected to a power source VCC. The emitters of the phototransistors 82, and 92 are respectively connected to a base of the transistors 83 and 93. The collectors of the transistors 83 and 93 are respectively connected to the input terminals XA and XB of the pointer control circuit 70.
As described, as the mouse moves along the X coordinate, the corresponding photogrid wheel also rotates, causing the phototransistors 82, 92 together with the transistors 83 and 93 to output a corresponding number of pulses to the input terminals XA, XB of the pointer control circuit 70, thus providing X displacement signal to the pointer control circuit 70. A Y displacement signal to the pointer control circuit 70 is similarly provided.
From the above structure and description, it is known that the precision for detecting the X displacement and the Y displacement depends on the cooperation of the light emitting diodes 81, 91 and the phototransistors 82, 92, i.e., the illuminations of the two light emitting diodes 81, 91 should be identical, and the gains of the phototransistors 82, 92 have to be identical. However, to obtain two phototransistors with identical gain requires cumbersome testing procedure. In addition, the illumination from the light emitting diodes 81, 91 has to be adjusted via a variable resistor 84 connected to the diodes 81 and 91.
Since the efficiencies of the light emitting diodes may be different thus affecting the precision of the detection of the phototransistors. Additionally, the obtaining manner of the power source VCC may affect the operative precision of the phototransistor. For example, a serial-output-type mouse obtains a DC power directly from a signal terminal of a serial interface RS-232C. However the signal levels for different computers are not exactly the same thus causing variation on the source voltage VCC and affecting the detecting precision of the phototransistors. Alternatively, the mouse may obtain power source from a computer connected thereto by using the same power supply of the computer. In this case, the precision problem is raised when the mouse is connected to a notebook computer. It is noted that the notebook computer uses batteries as its power source thus the precision of the mouse is decreased if the total voltage from the batteries reduces after long term use.
It is clear that the mouse suffers from an instability problem due to insufficient voltage from the power supply and also suffers from a precision problem due to parts difference such as the characteristic difference of the phototransistors and the light emitting diodes.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages of the conventional driving circuit.